A magnet-type rodless cylinder provided with cylinder holes formed in a cylinder tube, pistons disposed in the cylinder holes so as to move therein, and a slider disposed on the outer side of the cylinder tube and moves along the outer circumference of the cylinder tube, the pistons and the slider being magnetically coupled together, is known in the art.
In magnet-type rodless cylinders, generally, magnets (inner magnets) are arranged in the pistons and magnets (outer magnets) or magnetic material are arranged on the slider. Due to the attracting forces exerted between these magnets and/or magnetic material, the pistons and the slider are magnetically coupled together, and the slider follows the movement of the pistons.
There is known a magnet-type rodless cylinder having a plurality of cylinder holes and a plurality of pistons, in which all of the pistons are magnetically coupled with a single slider
Rodless cylinders have been disclosed, for example, in the following documents A to F.                Document A: JP-UM-A-4-113305        Document B: JP-A-4-357310        Document C: Japanese Utility Model Registration No. 2514499        Document D: JP-A-60-172711        Document E: U.S. Pat. No. 3,893,378        Document F: JP-A-9-217708        
Document A discloses a magnet-type rodless cylinder in which the cylinder tube and the pistons are formed in a flat shape in a transverse cross section in order to decrease the size of the device and to increase cylinder thrust.
Document B discloses a magnet-type rodless cylinder in which the cylinder tube and the pistons are formed in an elliptic shape, in an oval shape or in a symmetrically pear shape in a transverse cross section.
Further, document C discloses a magnet-type rodless cylinder in which two cylinder tubes each having a cylinder hole are arranged in parallel, and a single slider is provided so as to surround the pair of cylinder tubes.
Document D relates to a slit-tube-type rodless cylinder. Document D discloses a rodless cylinder in which two cylinder holes are formed in parallel in one cylinder tube with pistons disposed in the cylinder holes so as to move in the axial direction of the cylinders.
In the rodless cylinder of document D, the two pistons are mechanically coupled to a single slider via slits opened in the walls of the cylinder tubes and covered with sealing bands.
Document E also relates to a slit-type-rodless cylinder. Document E discloses a rodless cylinder in which the cylinder tube and the cylinder holes are of a rectangular shape in a transverse cross section, and the pistons are also formed in a rectangular shape in a transverse cross section corresponding to the shape of the cylinder holes.
Document F relates to a rod-type cylinder. The rod-type cylinder is provided with a rod connected to a piston extending in the axial direction, and the movement of the piston is transmitted to an external part of the cylinder tube through the rod. Document F discloses a rod-type cylinder in which two cylinder holes are formed in parallel in a cylinder tube.
FIG. 6 illustrates a magnet-type rodless cylinder 61 disclosed in document C.
The magnet-type rodless cylinder 61 of FIG. 6 has a pair of cylinder tubes 62 arranged in parallel with each other with cylinder tubes coupled and fixed together by end caps 67 provided on both ends of the cylinders.
Further, cylinder holes (not shown) are formed in the cylinder tubes 62, and pistons (not shown) are contained in the cylinder holes.
A slider 64 is disposed on the outer side of the cylinder tube 62 to surround both cylinder tubes 62.
Inner magnets are disposed in the pistons in the cylinder holes and outer magnets are disposed on the inner surface of the slider through which the cylinder tubes pass through. The two pistons and the single slider are magnetically coupled together by the attracting forces between the inner magnets and the outer magnets.
In the magnet-type rodless cylinder 61 of FIG. 6, working fluid such as compressed air is supplied into the cylinder holes in the cylinder tubes through the end caps 67 on both sides, whereby the two pistons move in the cylinder tubes in a synchronized manner. Therefore, the slider integrally coupled to the pistons by magnetic force moves on the outer side of the cylinder tubes following the movement of the pistons.
Generally, in magnet-type cylinders that are now being used, the cylinder tubes and the cylinder holes are of an exactly circular shape in cross section. Therefore, even when internal pressure acts on the tubes, the cross section of the tubes undergoes a uniform deformation (expansion), and stress acting on the tubes is uniform without producing partial deflection or concentration of stress.
However, when cylinder tubes having a flat (non-circular) sectional shape are used as disclosed in documents A and B, the cylinder holes also have a non-circular shape in cross section. Therefore, if internal pressure is excerted by the fluid, the tubes undergo a non-uniform deformation. When cylinder tubes having a non-circular shape in cross section are used, a stress concentration or partial deflection occurs on the tube, and both maximum stress and maximum deflection of the tube may become excessive.
To solve this problem, it is possible to increase the thickness of the tubes to enhance the rigidity of the tubes. If the thickness of the tubes is increased, however, it is necessary to increase the magnetic coupling force coupling the pistons to the slider. In this case, the required magnetic coupling force often is several times greater than the magnetic coupling force when tubes are used having a circular shape in cross section.
Because of this, magnet-type rodless cylinders having cylinder holes of a non-circular shape are difficult to be put into practical use.
On the other hand, the magnet-type rodless cylinders of document C solved the above problem by arranging two cylinder tubes each having an exactly circular shape cross section in parallel.
However, when a plurality of cylinder tubes 62 are used as disclosed in document C, the number of parts used for the magnet-type rodless cylinder increases. This causes an increase in the number of the assembling steps and an increase in the installation space of the cylinders.
Further, if the two cylinder tubes 62 are arranged close to each other in parallel as disclosed in document C, the inner magnets provided in the pistons in the cylinder tubes repel each other, and the pistons receive repulsive forces in an outward direction. Accordingly, the pistons are pushed against the inner walls of the cylinder holes due to the repulsive force, and the friction force between the pistons and the cylinder walls increases with an increase in the pressure of the contact surface between the pistons and cylinder walls. This results in an increase in the minimum operation pressure of the working fluid required for moving the pistons when supplying the working fluid into the cylinder. An increase in the minimum operation pressure of the working fluid causes a problem of decreased durability at various portions of magnet-type rodless cylinders.